The ability to dope polysilicon gates to different degrees allows one to adjust the work function of gate electrode materials to particular types of metal oxide silicon (MOS) transistors. It is desirable to adjust the work function of a gate electrode or gate, to be close to either the conduction band or the valence band of silicon, because this reduces the threshold voltage (Vt) of the transistor, thereby facilitating a high drive current at low operating voltages. Dual work function gates, for example doped polysilicon, are advantageously used in semiconductor devices, such as complementary metal oxide silicon (CMOS) transistor devices, having both pMOS and nMOS transistors. The use of doped polysilicon gates has become problematic, however, as the dimensions of gates and gate insulators alike have significantly reduced.
It is well understood that polysilicon gates can accommodate only a finite amount of dopants. This limitation can result in a depletion of gate charge carriers at the interface between the gate and gate dielectric, when the gate is biased to invert the channel. Consequently, the electrical thickness of the gate stack is substantially increased, thereby deteriorating the performance characteristics of the transistor.
In view of the shortcomings of doped polysilicon, metal gates are an attractive alternative to polysilicon because they have larger supply of charge carriers than doped polysilicon gates. When a metal gate is biased to invert the channel, there is no substantial depletion of carriers at the interface between the metal gate and gate dielectric. Accordingly, the transistor's performance is not deteriorated because the electrical thickness of the gate stack is not increased. In the manufacture of semiconductor devices, having independently adjustable dual work function metal gates has been troublesome, however.
Ideally, dual work function metal gates should be compatible with the type of device in which it will operate. However, during fabrication processes and due primarily to the thermal budgets involved, the work function of each of these metal gates may shift or drift either up or down, thereby changing the work function and consequently, device performance and also changing the Vt uniformity (i.e., the Vt from one device to another).
One of the more challenging aspects of gate electrode work function or threshold voltage control is controlling the gate electrode crystallographic texture. Most metals and metal compounds under consideration for use as metal gates are crystalline, and most crystalline metals have some variation of work function, and therefore, threshold voltage, for certain grain orientations. Thin metal layers may have fairly well or well-oriented grains, but they often are not single orientation. As a result, the work function of the resulting metal may vary from device to device, especially in the case of narrow lines, e.g., sub 30 nm. This non-uniformity can ultimately affect device performance, and is, therefore, undesirable in view of the stringent operating requirements that present day technologies are expected to meet.
Accordingly, what is needed in the art is a semiconductor device that avoids the disadvantages associated with the current manufacturing processes.